We will pursue a radically different approach to HIV vaccine design. Treatment of HIV-infected patients with single antiretroviral drugs often results in the emergence of variant, drug-resistant viruses. We have recently shown that cellular immune responses place similar selective pressure on the virus. We will design candidate vaccines by identifying those regions of the virus under intense selective pressure during acute SIV infection. We will sequence the entire virus at 4 and 16 weeks post-infection to determine cellular immune responses that are important in controlling early viral replication. Our hypothesis is that vaccine-induction of cytotoxic T lymphocyte (CTL) that exert selective pressure on the virus will reduce initial virus replication in macaques challenged with SIV. To address this hypothesis we first need to know which of the CTL responses control virus replication. In Specific Aim I we will sequence the entire SIV viral genome from 12 macaques at 2, 4 and 16 weeks post-infection with a molecularly cloned virus to identify regions of the virus, which have escaped. We will use an overlapping set of peptides spanning the entire SIVmac239 genome to map cellular immune responsesin these infected animals using ELISPOT assays. In Specific Aim IIwe will then vaccinate macaqueswith the regions of the virus that have escaped recognition by the immune response. We already have preliminary data suggesting that virus isolated 4 weeks post-infection has "escaped" from a strong CTL response. This suggests that the CTL responsible for selecting the escape variants had destroyed all cells actively producing the wild-type virus. We will, therefore, use epitopes in the wild-type virus that escape during acute infection to induce robust CTL. We predict that these vaccine-induced CTL responses will reduce the initial viremia, and prevent escape mutant generation, thereby facilitating the development of strong host immune responses. The results of this proposal utilizing rhesus macaques challenged with SIV will have direct relevance to the rational design of a vaccine for HIV. If vaccination with regions of the virus that escape during the acute phase reduce initial viremia in macaques, then we can use analogous regions in HIV in vaccination regimens. Indeed, a CTL epitope-based vaccine is already in Phase I trials in Oxford and Kenya under the direction of Dr. McMichael. Thus, results from our studies will be important in the eventual design of this vaccine.